1. Field of the Invention
The present invention relates to a cutting tool and a production method therefor. More particularly, the invention relates to a cutting tool having a sensor circuit for detecting the end of the service life of the cutting tool and to a production method therefor.
2. Description of Related Art
The end of the service life of a cutting tool having a cutting edge defined along an intersection ridge between a rake face and a flank is generally judged on the basis of the amount of wear of the flank. For highly accurate machining, it is imperative to speedily determine the amount of the wear of the flank on an in-process basis during a cutting operation. However, it is very difficult in view of a machining environment to directly observe the wear of the tool during the machining.
Therefore, the determination of the amount of the wear of the tool is generally achieved by interrupting the machining, detaching the tool and observing the tool by means of an optical microscope. Alternatively, where it is desired to determine the wear amount on an in-process basis during the machining, a phenomenon (a change in cutting power and vibrations) occurring due to the wear of the tool is detected by means of a sensor provided in the vicinity of a machining point on a machine tool, and a detection signal is processed for estimation of the wear amount.
However, the quantitative determination of the wear amount on an in-process basis is difficult, failing to provide satisfactory sensitivity and reliability.
There is also proposed a method in which the end of the service life of the cutting tool is automatically judged by detecting the amount of the wear of the cutting edge of the cutting tool. In this method, a self-diagnosis is made on the end of the service life of the tool by detecting the amount of the wear of the cutting tool. Where this method is applied to an electrically conductive tool, an electrically conductive line is embedded in an insulation layer, and the wear limit of the tool and the cut-off of the electrically conductive line are detected on the basis of a signal outputted when the electrically conductive line is cut off in a cutting operation (see Japanese Unexamined Patent Publication No. 62-88552 (1987)).
There is also provided a method in which a cutting tool including an electrically conductive substrate having a surface coated with an insulative aluminum oxide film is provided, and a voltage is applied between a workpiece and the tool for detection of the end of the service life of the cutting tool. That is, the end of the service life of the tool is judged by detecting a flow of an electric current when the aluminum oxide film is worn out (see Japanese Unexamined Patent Publication No. 59-81043 (1984)).
In these methods, the insulation state of the insulation layer is very important. In the method proposed in the Japanese Unexamined Patent Publication No. 62-88552, an aluminum oxide film is formed as the insulation layer by a CVD method or a PVD method. Where a cemented carbide substrate or a cermet substrate is coated with the aluminum oxide film which has a thermal expansion coefficient different from that of the substrate by the CVD method, the aluminum oxide film suffers from minute cracks extending to the substrate after the coating. When an electrically conductive sensor film is formed on the aluminum oxide film, the electrically conductive substrate and the electrically conductive sensor film are shorted through the cracks.
Where the aluminum oxide film is formed by the PVD method, the aluminum oxide film suffers from defects such as pinholes extending to the substrate, and the electrically conductive substrate and the electrically conductive sensor film are shorted through the pinholes. Further, an electrically conductive unreacted product may remain in the insulation layer depending on the state of the substrate. These problems make it very difficult to produce a sensor circuit having a stable sensor function.
In the method proposed in Japanese Unexamined Patent Publication No. 59-81043, the aluminum oxide film needs to have a thickness of at least 10 μm to 100 μm to ensure a satisfactory insulation property. This is because minute cracking and chipping are present in the aluminum oxide film.
This is conceptually illustrated in FIG. 1. In FIG. 1, the electrically conductive film, the insulation layer and the electrically conductive substrate are denoted by reference numerals 1, 2 and 3, respectively. There are further shown a defect 4 such as a crack or a pinhole and a defect 5 such as electrically conductive grains. If the defects 4, 5 (e.g., cracks, pinholes and electrically conductive grains) possibly causing a short circuit are present in the insulation layer 2, the electrically conductive substrate 3 and the electrically conductive film 1 are shorted via the defects 4, 5 when the electrically conductive sensor circuit is produced. This hinders proper function of the sensor circuit, and reduces the reliability of the sensor circuit.
To cope with the aforesaid problems, there have been proposed various types of sensor-incorporating tools, but none of them have been put to practical use. That is, small-volume production of such tools is possible on an experimental basis, but the tools are not feasible as cutting tools.
On the other hand, where the thickness of the insulation layer is increased for elimination of the influences of the cracking and chipping present in the insulation layer as proposed in Japanese Unexamined Patent Publication No. 59-81043, the tool has a deteriorated cutting performance and a poorer chipping resistance.
In view of the foregoing, it is an object of the present invention to provide a cutting tool which is highly reliable in determining the end of the service life thereof, and to provide a production method therefor.